The invention uses "pulse swallow" techniques, for minimizing the need for high-speed logic circuitry. The term "pulse swallow" implies that, in a stream of cyclically recurring pulses, pulses are periodically "swallowed" or removed in order to reduce a need for high-speed logic.
Conventional digital frequency dividers of this kind, are disclosed by Nichols et al. in an article entitled "Pulse Swallowing", which was published in the EDN magazine for Oct. 1, 1970, pp. 39-42, and in Motorola Semiconductor Products, Inc., "Phase-Locked Loop Systems Databook", August 1973, pp. 1-17. Usually, these dividers comprise a two-modulus prescaler (or counter), and first and second programmable counters. The two-modulus prescaler selects one of two possible frequency-division factors P and P+1 (where P is an integer) in response to a control signal, thereby frequency dividing an input signal. The first programmable counter frequency divides the output of the prescaler by a factor of N (where N is an integer) to generate a desired frequency-division output. The second programmable counter frequency divides the output of the prescaler by a factor A (where A is an interger) to generate the control signal. This divider is suitable for use in a phase-locked loop (PLL) frequency synthesizer or the like, for example.
In such a digital frequency divider, the control signal for the prescaler must be fed back within a transmission delay time, which is equal to a cycle (t.sub.c) of the output pulse of the prescaler. In other words, the time cycle (t.sub.c) must be greater than the sum of the set-up time (t.sub.ps) required for switching the frequency-division factor of the prescaler, plus the propagation time (t.sub.A) of the second programmable counter. This time relationship problem can be solved by enlarging either the factor P of the prescaler, or the cycle t.sub.c. However, such an enlarged time factor causes a narrowing of the applicable range of the frequency divider. Moreover, this solution can only expand the factor P, and can not reduce the propagation time of the feedback loop.